Rodríguez-León et al. Nanoscale Research Letters 2013, 8:318 http://www.nanoscalereslett.com/content/8/1/318 NANO EXPRESS Open Access Synthesis of silver nanoparticles using reducing agents obtained from natural sources (Rumex hymenosepalus extracts) Ericka Rodríguez-León1, Ramón Iñiguez-Palomares4*, Rosa Elena Navarro3, Ronaldo Herrera-Urbina2, Judith Tánori3, Claudia Iñiguez-Palomares5 and Amir Maldonado4 Abstract We have synthesized silver nanoparticles from silver nitrate solutions using extracts of Rumex hymenosepalus, a plant widely found in a large region in North America, as reducing agent. This plant is known to be rich in antioxidant molecules which we use as reducing agents. Silver nanoparticles grow in a single-step method, at room temperature, and with no addition of external energy. The nanoparticles have been characterized by ultraviolet- visible spectroscopy and transmission electron microscopy, as a function of the ratio of silver ions to reducing agent molecules. The nanoparticle diameters are in the range of 2 to 40 nm. High-resolution transmission electron microscopy and fast Fourier transform analysis show that two kinds of crystal structures are obtained: face-centered cubic and hexagonal. Keywords: Silver nanoparticles; Rumex hymenosepalus; Antioxidants; Electron microscopy; Green synthesis Background of nanoparticles of several materials: metallic [25-27], di- The synthesis of nanomaterials is of current interest due electric [28,29], semiconductor [30,31], and magnetic to their wide variety of applications in fields such as elec- [32,33]. However, the intensive use of solvents and syn- tronics [1-4], photonics [5-7], catalysis [8-10], medicine thetic reactants is harmful for the environment. For this [11-15], etc. Most of the applications are due to the fact reason, it is very desirable to devise alternative, ‘green’ that matter at the nanometer scale has different properties methods of nanomaterial preparation that use environ- as compared with the bulk state. For this reason, many re- mentally friendly reactants. The silver nanoparticles search groups around the world are trying new methods obtained by the green synthesis method are candidates to of synthesis of different materials at the nanoscale. One be used in biological systems. goal is to control the size and shape of atomic clusters or In the case of silver particles, the nanocrystals are usu- nanoparticles and their ordering in 1D, 2D, or 3D arrays. ally grown from Ag+ solutions. The silver ions come In particular, silver nanoparticles have been used with from a salt like silver nitrate (AgNO3). The ions are first promising results as bactericides [16-21], antimicotics reduced to atoms by means of a reducing agent. The [22], and anticancer agents [21,23,24]. obtained atoms then nucleate in small clusters that grow Several methods have been devised in order to prepare into particles. Depending on the availability of atoms, metallic nanoparticles. For instance, one of the current which in turn depends on the silver salt to reducing methods crystalizes nanoparticles in microemulsions, agent concentration ratio, the size and shape of the using a variety of chemicals as precursors and large nanoparticles can be controlled. In this method, two ele- amounts of surfactants as stabilizing agents. The different ments are needed for the nanoparticle grow: a silver salt preparation methods have been successful in the synthesis and a reducing agent [34,35]. On the other hand, in recent times, there is a growing * Correspondence: [email protected] interest in the synthesis of metal nanoparticles by ‘green’ 4Departamento de Física, Universidad de Sonora, Apartado Postal 1626, methods. For this purpose, biomass or extracts of differ- Hermosillo, Sonora 83000, México ent plants have been tried with success as reducing Full list of author information is available at the end of the article © 2013 Rodríguez-León et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Rodríguez-León et al. Nanoscale Research Letters 2013, 8:318 Page 2 of 9 http://www.nanoscalereslett.com/content/8/1/318 agents. For instance, in the literature, there are reports spectroscopy (Perkin Elmer Lambda 20 spectrophotom- of the synthesis of silver or gold nanoparticles using eter, PerkinElmer, Waltham, MA, USA) and proton nu- extracts of different plants [17-20,23,24,36-49]. The clear magnetic resonance (1H NMR) experiments with a present work is part of this new line of research. Bruker Avance 400 apparatus (Bruker AXS Inc., Madison, In our study, the reducing agent comes from extracts WI, USA) operating at 400 MHz, at 25°C. For the NMR of Rumex hymenosepalus, which is a plant rich in poly- experiments, a portion of the Rh extract was concentrated phenols. In the literature, there is no report on the syn- on a rotary evaporator at 37°C and dried under vacuum. thesis of nanoparticles using extracts from this plant. It The resulting dark brown solid was washed three times is a vegetal species abundantly present in North Mexico with 100 ml of tetrahydrofuran (Aldrich 99.9% purity) and and in the south of the USA. In Mexico, it is collected, purified using a glass filter. The filtrate was evaporated dried, cut, and packed for selling to the public. This and dried under vacuum. With the solid, obtained NMR plant, also known as canaigre dock or wild rhubarb, can tubes were prepared in deuterated dimethyl sulfoxide be of interest for green synthesis because it contains a (DMSO-d6). The internal reference was tetramethylsilane. large amount of natural antioxidants. Among the anti- For the nanoparticle synthesis, we have prepared one oxidant molecules this plant contains, polyphenolic solution of AgNO3 in water; the concentration was compounds, like flavan-3-ols (tannins) and stilbenes, are 0.1 M. Different volumes of this solution have been found in large quantities. These molecules are poten- mixed with a fixed volume of the Rh extract (VRh = tially strong reducing agents due to their numerous OH 200 μl); the total volume of each sample was adjusted to groups that promote their antioxidant activity [50,51]. 4mlbyaddingthenecessaryamountofethanolin In this paper, we present results on the synthesis of silver order to prepare samples with different AgNO3 concen- nanoparticles using extracts of the plant R. hymenosepalus trations:2.5,5,7.5,10,and15mM.Theextractconcen- (Rh extracts) as reducing agent in aqueous silver nitrate tration was 5% v/v in all the samples. solutions. We have extracted the antioxidant fractions For each AgNO3 concentration, the reduction reaction from dried roots of the plant. We have characterized has proceeded along 96 h. The experiment was per- the resulting nanoparticles by transmission electron mi- formed under regular, indoor illumination. The samples croscopy (TEM) and ultraviolet-visible (UV-Vis) spec- were analyzed every 24 h by visual inspection and UV- troscopy. To the best of our knowledge, this is the first Vis spectroscopy. The nanoparticles have been observed report in the literature on nanoparticle synthesis using with TEM using a Jeol 2010 F apparatus (JEOL Ltd., extracts of this plant. Akishima-shi, Japan) operating at 200 kV. We have de- posited 10 μl of the nanoparticle suspension on a Methods formvar-carbon coated copper TEM grid (300 Mesh). We have purchased dried, slice-cut roots of R. The sample was vacuum-dried for 24 h before observa- hymenosepalus in a local convenient store (Comercial tion. From the TEM micrographs, the size distribution Zazueta, Hermosillo, Mexico); we present a picture of was obtained, as well as the average diameter. The the dried roots in the Additional file 1: Figure S1. Etha- chemical composition of the nanoparticles has been nol (99%) and silver nitrate (AgNO3 99%) are from obtained with energy dispersive X-ray spectroscopy Sigma-Aldrich(St.Louis,MO,USA).FortheUV-Vis (EDS) using a Bruker Quantax 200 detector (Bruker calibration curves, we have used epicatechin (98%) and AXS Inc., Madison, WI, USA). The crystal structure of epicatechin gallate (95%); both molecules were pur- the nanoparticles has been obtained from high- chased in Sigma-Aldrich. We have used ultra-purified resolution TEM (HR-TEM) experiments and from the water (Milli Q system, Millipore, Billerica, MA, USA). corresponding fast Fourier transform (FFT) plots. In order to prepare the plant extract, we have put 15 g of a dried R. hymenosepalus sample in a flask, and then, Results and discussion we have added 100 ml of an ethanol/water solution (70:30 The extraction procedure from dried R. hymenosepalus v/v). The flask was stored at room temperature (T = 25°C), roots yielded a dark liquid which we examined spectro- and the extraction was allowed to proceed during sev- scopically. The UV-Vis and NMR experiments confirm eral days; the visual appearance of the liquid was moni- the presence of polyphenols in the Rh extract. The absorb- tored on a daily basis. Its color changed from a light red ance of the sample presented a peak in the range of 277 to the first day to a darker brown. After 15 days, the extrac- 280 nm as characteristic of polyphenol molecules. On the tion was considered complete since no change in the color other hand, the 1H NMR proton spectra display a wealth was noticeable. The sample was then filtered, and the of peaks characteristic of plant extracts (Additional file 1: resulting liquid is the Rh (R. hymenosepalus) extract that Figure S2). We have identified some of these signals as has been used as reducing agent in the nanoparticle syn- corresponding to polyphenol molecules [52] (Additional thesis. The Rh extract has been characterized by UV-Vis file 1: Figures S3 and S4). In particular, some peaks Rodríguez-León et al.